1. Field of the Invention
The field of the invention is free-venting pipe for conducting oil and gas or other fluids in subsea and offshore operations and method of manufacturing the pipe.
2. Description of the Related Art
This is an improvement on the invention disclosed in U.S. Pat. No. 6,804,942, which is herein incorporated by reference. When the pipe disclosed therein is used to transport compressed gas, or fluids containing gas, some gas can permeate through the inner core or the pipe over time and gather between the inner core and extrusions causing a steady rising inward pressure on the core. The oil and gas industry and those skilled in the art generally refer to spaces between the inner core and the extrusions as annulus regions. Although not in itself a problem under normal circumstances, pressure that builds up in the annulus regions can cause a problem if the pressure in the bore of the pipe is released more quickly than the pressure between the layers. When that happens, the inner core can collapse due to the adverse pressure differential and its poor hoop strength. This problem can become more serious as the depth of water in which pipes are used to transport oil and gas increases, along with the increased ambient pressures at deeper water depths.
Pipe collapse has happened in the past with prior art pipes that have steel based flexible pipes and therefore prior art pipes used for gas transmission are normally constructed using a central interlocking metallic carcass under the inner core. The metallic carcass provides sufficient radial strength to withstand any collapse forces generated by the scenario described above but the overall diameter of the pipe necessarily requires an increase of twice the thickness of the carcass. Furthermore, the inclusion of a carcass increases each reinforcement layer for the same performance because the reinforcement layers have to be wound on larger diameters. An example of such a configuration is shown in U.S. Pat. No. 6,978,806.
Some degree of protection can be provided to pipes by relieving the pressure between the two extrusions through a valve or valves in the end fittings but this is insufficient for all circumstances and collapse of the inner core may still occur.
The problem has also solved by Michael J. Bryant, the inventor of the current invention, in pending, published US Patent Publication Number US-2006-0249215-A1, wherein a method is taught to prevent the collapse of the core by embedding a polymer in laterally spaced openings formed in reinforcing tapes that surround the core. The embedded polymer is then bonded to the core to strengthen the hoop strength of the core to prevent its collapse.